The present invention relates to a dispersion-type electroluminescence element (dispersion-type EL element) for use in various electronic appliances as back lighting for the display section or the operating section.
An increasing number of electronic appliances, which have been diversifying into quite a number of different configurations, incorporate a back lighting tool behind their liquid crystal display panels or operating sections in order to facilitate an easier handling or an easier recognition by the eyes in the darkness. Dispersion-type EL elements have been widely used as the back lighting tool.
A conventional dispersion-type EL element is described in the following with reference to the drawings.
In the drawings, the illustrations have been shown magnified in the direction of thickness for the sake of easier description of the structure.
FIG. 16 is a cross sectional view of a conventional dispersion-type EL element. On one of the surfaces of a flexible light-transmitting insulation film 1 made of polyethylene terephthalate or the like material, a light-transmitting electrode layer 2 of indium tin oxide (hereinafter referred to as ITO) is formed through a sputtering process or an electron beam method. On top of the electrode layer 2, a luminescence layer 3 comprising a fluorocarbon rubber, cyano- group resin, or the like dielectric resin having a high permittivity dispersed with zinc sulfide or the like fluorescent powder as the luminous body, a dielectric layer 4 of dielectric resin having a high permittivity dispersed with barium titanate or the like dielectric powder, a back electrode layer 5 composed of silver, carbon-resin group or the like conductive material connected with the dielectric layer 4, and an insulation layer 6 composed of epoxy resin, polyester resin or the like material are formed one layer after the other in the order by a printing process.
Wiring patterns 7A, 7B composed of silver or a conductive material of carbon-resin group are connected at the end portion to the light-transmitting electrode layer 2 and the back electrode layer 5, respectively.
When a dispersion-type EL element of the above-described structure is incorporated in an electronic appliance and an AC voltage is provided from a circuit (not shown) of the appliance on the wiring patterns 7A and 7B, which being connected respectively with the light-transmitting electrode layer 2 and the back electrode layer 5, the luminescence layer 3 of the dispersion-type EL element is driven to generate light. The light illuminates a display window, a liquid crystal display panel, etc. from behind. Thus, the display or an operating section can be easily recognized or identified even in a dark operational environment.
Color of a light to be emitted from a dispersion-type EL element is determined by a kind of fluorescent powder dispersed in the luminescence layer 3 made of a dielectric resin having a high permittivity. The luminescence color can be converted into a color other than the intrinsic color of the fluorescent powder, by dispersing a fluorescent dye or a fluorescent pigment in the dielectric resin having a high permittivity, or by tinting the insulation film 1.
In a dispersion-type EL element having the above-described conventional structure, however, only a single color is available although a luminescence color can be converted into other color by dispersing a fluorescent dye or a fluorescent pigment in the dielectric resin having a high permittivity forming the luminescence layer 3, or by tinting the insulation film 1. When a plurality of luminescence colors are needed, a plurality of dispersion-type EL elements have to be installed in an electronic appliance. This incurs an increased number of parts in an appliance, which leads to an additional cost and time for the fabricating operation. Thus the total cost goes higher.
Another conventional dispersion-type EL element is shown in FIG. 17. On the upper surface of a light-transmitting insulation film 101, a light-transmitting electrode layer 102 of ITO or the like material is provided in the form of thin film by a vacuum sputtering process or the like method. On top of the electrode layer 102, a luminescence layer 103 comprising a cyano-group resin or a fluorocarbon rubber group resin having a high permittivity dispersed with granular fluorescent powder such as a copper-doped zinc sulfide, etc., and a dielectric layer 104 comprising a synthetic resin of the same group as the material of luminescence layer 103 dispersed with barium titanate or the like powder of high permittivity are formed respectively in the order by a coating process. Further on top, a back electrode layer 105 composed of a paste of silver-resin group or a carbon-resin group material and an insulation layer 106 for protecting the back electrode layer 105 from contacting with outside element are formed respectively. And then, an outlet electrode 107 of the light-transmitting electrode layer 102 and an outlet electrode 108 of the back electrode layer 105 are formed respectively. When an AC voltage is applied between the outlet electrode 107 and the outlet electrode 108, the fluorescent powder being dispersed in the luminescence layer 103 is driven to produce a plane luminescence at the light-transmitting insulation layer 101 side.
With the above-described structure as the basis, a conventional dispersion-type EL element (Japanese Patent Publication No.60-130097) comprises a light-transmitting electrode layer 109 disposed in the form of a number of stripes, as shown in FIG. 18(a). The electrode stripes in the odd number rows are connected together at one end, while those in the even number rows are connected together at the other end; thus, the light-transmitting electrode layer 109 is formed of two comb-shape electrodes 110 and 111 integrated into one entity without making mutual contact to each other. A luminescence layer 112 comprising two different luminescence colors is provided on the comb-shape electrodes 110, 111 in an arrangement where the luminescence color 112A is located on the odd number rows, while the luminescence color 112B is on the even number rows, as illustrated in FIG. 18(b). A multi-color luminescence is made available by applying independent voltages on two respective comb-shape electrodes 110, 111.
However, in a conventional dispersion-type EL element of the above-described structure, where two kinds of luminous bodies 112A, 112B composed of synthetic resin dispersed respectively with different fluorescent powders for producing different luminescence colors are provided by a screen printing process, or the like process, in the form of stripes one after the other on a location corresponding to respective comb-shape light-transmitting electrodes 110, 111, it is difficult to provide the stripes of a small line-width precisely into a fine-pitch pattern because fluorescent powders generally have a relatively large grain diameter of approximately 30 xcexcm in average. If the stripe lines are formed in a rough-pitch pattern, the luminescence would appear to the eyes in a striped pattern rather than a plane luminescence when a voltage is applied on either one of the light-transmitting electrodes 110, 111 for producing a single-color luminescence. Thus the luminescence can hardly be recognized as a plane luminescence.
Furthermore, because of the large grain diameter of the fluorescent powder, thickness of the luminescence layer 112 is great and the surface condition is bumpy. When providing the luminous bodies 112A, 112B of two different colors alternately in a stripe form through a printing process, if there is a small deviation in the dimensions edges of the adjacent layers of different colors would readily be overlapped and the layer thickness of the overlapped portion increases, which makes the surface condition even bumpier. Then the printing of dielectric layer and back electrode layer on the luminescence layer would become difficult. Also, the electrode-to-electrode distance formed by the light-transmitting electrode layer and the back electrode layer becomes widely varied from place to place; which results in an uneven electrode-gap between the light-transmitting electrode layer and the back electrode layer, consequently an uneven luminescence would arise.
The present invention addresses the above-described drawbacks, and aims to offer a dispersion-type EL element that is capable of providing multiple colors in a homogeneous plane luminescence without accompanying an outstanding striped-appearance. More advantages of the dispersion-type EL element in accordance with the present invention include that mounting of the EL element on an electronic appliance is easy and that the manufacturing cost is low.
A dispersion-type EL element of the present invention is formed of a plurality of light-transmitting electrode layers and a plurality of luminescence layers composed of a dielectric resin having a high permittivity dispersed with fluorescent powder, each one of the respective layers being provided alternately one after the other over the whole region, or in a certain specific region, of one of the surfaces of a light-transmitting insulation film, and a back electrode layer is formed by a printing process on the last layer of the luminescence layers. In other example, two light-transmitting electrode layers having a comb shape of fine lines without making contact to each other, each of the electrode layers being capable of having different voltages independently to each other, are formed on one surface of a light-transmitting insulation film, and a luminescence layer of a single luminescence color is provided on the electrode layers; while, on the other surface of the light-transmitting insulation film, a color-conversion layer of a fine-teeth comb shape is provided in a location corresponding to at least one of the two light-transmitting electrode layers having a comb shape of fine lines, and a light diffusion layer is formed covering the outer surface.
In accordance with the above-described structure of the present invention, a dispersion-type EL element that is capable of providing several kinds of luminescence colors can be offered at a low cost. The plane luminescence provided by the EL element in a plurality of colors is homogeneous and stripes can hardly be recognized by the eyes in a normal operating environment.